1.与有或没有形容词修饰的抽象名词前，表示具体一个“人或物”。

　　He received a good education.

　　Thank you, Jim. You have done a kindness.

　　Many people agreed that a knowledge of English is a must in international trade today.

　　The meeting held yesterday was a success.

　　To advise him is a waste of time.

　　2.偶尔与专有名词连用，表示某种特定意思。表示类似的一个或某一个(a certain)

　　a Mr Black a living Lei Feng

　　A Mr. Smith wants to see you.

　　3.与形容词最高级连用，表示强调，不表最高级。

　　The Huanghe River is a longest river in China.

　　The story is a most interesting one.

　　4.用在有形容词修饰的一日三餐、季节和日期前。

　　Before I go to work every morning, I’ve a light breakfast.

　　The traffic accident happened on a Sunday to the end of the July.

　　5.用于序数词前，表示“再一，又一”而不表顺序。

　　How important it is to learn a second language.

　　The fisherman cast a third net.

　　6.表示类别，有any, per之意，a(n)不必译成汉语，此时a(n)可改为the或不用冠词(但必须用名词复数形式)。

　　His income is one thousand yuan a month.

　　A camel is a useful animal. = Camels are useful animals.

　　7.不定冠词用于一些原来是动词的名词前。

　　Let’s have a look around the playground after supper.

　　have a rest / walk / swim / talk … (have = take)

　　give a shout / whistle / loud laugh …

　　make a drive / move …

　　8.用在同源宾语中。

　　Now we are living a happy life.

　　Last night I dream a terrible dream.